law enforcement says that it appears dzhokhar and his older brother tamerlan was by i ideology. chilling new details in a ten-page fbi affidavit. a few minutes after the surveillance video captured the brothers at the marathon they split up according to court documents. tarrer lan heads to the finish line as his younger brother blends in with the crowd. dzhokhar places his backpack on the ground. at 2:50 the first bomb explodes. according to the affidavit, virtually every head turns, stares in that direction in apparent bewilderment and alarm, dzhokhar appears calm walk as way without his backpack seconds later, it detonates in the exact spot where he was landing. during thursday's getaway, one of the brothers bragged, did you hear about the boston explosion? i did that. and dzhokhar's roomdartmouth, key evidence. a black jacket andhat. matching the clothing he was wearing at the scene. law enforcement sources say there are clear signs that tamerlan, the older brother, had become increasingly radicalized, motivated by jihadi websites. they picked the boston marathon and boylston